The invention relates to a drill bit, having a front side and having a circumferential surface adjoining the front side.
Drill bits of the type considered here are known. In the region of their front side they have a main cutting edge, which comprises a transverse cutting edge intersecting a center axis of the drill bit and, adjoining the transverse cutting edge, main cutting-edge regions extending toward the circumferential surface. Provided in the region of the circumferential surface are three lands, of which two directly adjoin the radially outer ends of the main cutting-edge regions. A third land is located between these two lands. During the machining of a workpiece, the drill bit is supported on a drilled hole surface via the lands, and is thereby guided. The lands also serve to prevent vibration and to prevent the drill bit from drifting out of the axis of rotation. In the case of very accurately produced twist drill bits used to produce drilled holes whose diameter corresponds to that of the drill bit, high compressive stresses occur at the lands, because a lubricating film is scarcely able to form. The result of this is that there can be a very great increase in the cutting forces and in the temperature, which can result in a short service life of the drill bit and also in alterations of the workpiece material. Since the drill bit expands because of the heating, the compressive stresses and the problems associated therewith are increased further. During the spot-drilling operation, the guide lands are subjected to additional loading if the drill bit becomes displaced laterally relative to its center axis or axis of rotation. The lands seek to hold the drill bit in its off-center position, while the drill bit is being forced back into the axis of rotation because of the design of the main cutting edges. Frequently, this causes drilled holes to become out-of-round, which results in additional loading and in wearing of the lands. The capacity to regrind the drill bit is also considerably restricted.
To counter these problems, twist drill bits have been created having additional guide lands, which are instrumental in increasing the roundness of the drilled hole. They have the disadvantage, however, that they cannot compensate for relatively large displacements of the drill bit out of the axis of rotation, and are therefore actually disadvantageous with respect to the service life.